The information on the outpatient expenditure of patients with oromaxillofacial cancer is minimal. This study aimed to compare the average annual expenditure on dental treatment for these patients 5 years before and 5 years after oromaxillofacial cancer diagnosis. In this study, 7731 patients who received oromaxillofacial cancer diagnosis in 2005 were selected from the Registry of Catastrophic Illness Database as the case-cohort. In the control cohort, 38,655 people without cancer were selected from the National Health Insurance Research Database, with the case–control ratio being 1:5. All participants were observed for 5 years before diagnosis and 5 years after diagnosis. The conditional logistic regression model was used to determine the odds ratios of annual expenditures incurred by participants in the case-cohort. The measurement results indicated that in the oromaxillofacial cancer cohort, the average annual dental expenditure levels at 1, 2, 3, 4, and 5 years after diagnosis were US $97.34, US $77.23, US $109.65, US $128.43, and US $128.03 and those at these years before diagnosis were US $37.52, US $32.10, US $31.86, US $29.14, and US $29.35, respectively. In conclusion, the average annual expenditure on the dental treatment of oromaxillofacial cancer patients after five years of diagnosis was increased compared to five years before diagnosis.